[Q-e-developers] ibrav = +/- 9

[stefano de gironcoli]

On 06/11/2015 16:21, Lorenzo Paulatto wrote:
> Dear All,
> there is a small bug or a problem with the documentation of ibrav= -9
>
>
> The Doc says:
>
> 9          Orthorhombic base-centered(bco) celldm(2)=b/a
>                                             celldm(3)=c/a
>      v1 = (a/2, b/2,0),  v2 = (-a/2,b/2,0),  v3 = (0,0,c)
> -9          as 9, alternate description
>      v1 = (a/2,-b/2,0),  v2 = (a/2,-b/2,0),  v3 = (0,0,c)
v1 = v2 ?
i think the code is right and the doc is wrong... or both are wrong :-)
stefano
>
> However the code says:
>
>       IF ( ibrav == 9 ) THEN
>          !   old PWscf description
>          a1(1) = 0.5d0 * celldm(1)
>          a1(2) = a1(1) * celldm(2)
>          a2(1) = - a1(1)
>          a2(2) = a1(2)
>       ELSE
>          !   alternate description
>          a1(1) = 0.5d0 * celldm(1)
>          a1(2) =-a1(1) * celldm(2)
>          a2(1) = a1(1)
>          a2(2) =-a1(2)
>       END IF
>
> Which means:
> -9          as 9, alternate description
>      v1 = (a/2,-b/2,0),  v2 = (a/2,b/2,0),  v3 = (0,0,c)
>
>
> I.e the sign of the y-component of v2 is the opposite. 	
>
> I'm writing here because I do not know which of the two is correct. Thanks to
> G Radtke for bringing this to my attention (He thinks that the Doc is correct
> and the code is wrong)
>
> LP
>
>